WESTERN AUSTRALIA

CLIMATE DRIVERS IN WESTERN AUSTRALIA

In Western Australia, Ridgy is great at blocking rain-bearing fronts. He lets cold fronts bearing rain through much more in winter in the south, and brings fine and dry weather in summer. When Ridgy heads north, he affects when the monsoon starts.

Indy herds moisture from the Indian Ocean to some parts of the state, bringing drier or wetter years depending on whether he’s feeling positive or negative.

Sam loves to play in the south-west – herding up cold fronts from the southern ocean, affecting rainfall in autumn and winter, and sometimes spring, bringing rainfall triggers to southern WA.

Enso herds moisture from the Pacific Ocean which can sometimes affect northern parts of WA.. When there’s an El Nino, he causes less rainfall and fewer tropical cyclones. During La Ninas, he chases greater amounts of moist tropical air across Australia and causes more tropical cyclones.

Mojo mainly affects northern Australia, and sends a wave of weather across the Indian Ocean which can create cyclones and bring widespread rain. He’s most active from October to April.

greater exposure of stock and crops to heat-related stress and disease

earlier ripening and reduction in grape quality

less winter chilling for fruit and nuts

southern migration of some pests

potential increase in the distribution and abundance of some exotic weeds

TEMPERATURE

Temperature projections for Western Australia are for continued warming over the coming decades.

The projection is for a 1.0°C increase over 1990 climate by 2030 in southern and coastal parts of Western Australia, and for a 1.5°C increase in inland Western Australia. Under a low greenhouse gas emission scenario (best case), projection is for a 1.5°C increase over 1990 climate by 2070, and under a high greenhouse gas emission scenario (worst case), a 3.0°C increase is projected.

Less warming is expected along the coast and southern regions than in the rest of Western Australia. Summer is expected to be warmer in line with the projected average increase. Spring is likely to be warmer by slightly more than the average projected increase. Autumn and winter are likely to be warmer by slightly less than the average projected increase.

Scientists have more confidence in the projections for mean temperature than in those for rainfall. They have more confidence in the projections for 2030 than in those for 2070.

RAINFALL

Rainfall projections indicate a drying trend for the whole state, apart from the Kimberley region. Rainfall projections for Western Australia are more mixed than the projections for temperature. We can expect a drying trend during winter and spring. Summer and autumn rainfall changes are less certain.

Climate models consistently predict reduced winter and spring rainfall for south-west Western Australia. When a range of models predict the same result for a given region, it increases confidence in the prediction.

The projection is for a 2-5% decrease in annual rainfall in 2030 relative to the 1990 climate. Under a low greenhouse gas emission scenario (best case), projection is for a 5-10% decrease in annual rainfall by 2070 when compared to 1990, and under a high greenhouse gas emission scenario (worst case), a 10-20% decrease is projected.

Scientists have more confidence in the projections for temperature than in those for rainfall. They have more confidence in the projections for 2030 than in those for 2070.

RUNOFF AND STREAMFLOW

The combination of projected warming and less rainfall has serious implications for runoff and water storage. By 2030, streamflow into dams in the south-west is projected to decline by up to 40 per cent relative to historical average flows.

DROUGHT AND EXTREME RAINFALL

Most models predict that potential evapotranspiration will increase over Western Australia. Evapotranspiration is the combination of evaporation from soil and water surfaces, and transpiration from vegetation. When these changes are combined with the projected declines in rainfall, an increase in aridity and drought occurrence is likely.

Climate projections show an increase in daily precipitation intensity over much of the state, except the far south-west and central parts. The number of dry days is expected to increase significantly everywhere except for the Kimberley region. Future rainfall patterns for many areas, including the northern part of the wheatbelt, will have longer dry spells interrupted by heavier precipitation events.